1. Field of the Invention
The present invention relates to an improved copolyester fiber. More particularly, the present invention relates to an improved copolyester fiber capable of being dyed with cationic dyes and having a satisfactory mechanical strength and an excellent brightness.
2. Description of the Related Art
It is known that polyester resins have various excellent chemical and physical properties, and thus are useful for fibers (filaments) and films. However, it is also known that the polyester fibers exhibit a poor dyeability and are not easily dyed with dyes other than disperse dyes.
It is further known that, when the polyester fiber material dyed with a disperse dye is coated with a polymeric resin material, the resin-treated polyester fiber material exhibits a decreased fastness to laundering, ironing, and rubbing.
Due to the above-mentioned disadvantages, various attempts have been made to enhance the dyeability of polyester fibers.
In one such attempt, Japanese Examined Patent Publication (Kokoku) No. 34-10497 disclosed a copolyester fiber capable of being dyed with cationic dyes and produced from a copolyester resin containing an additional copolymerization component consisting of an isophthalic acid derivative having a metal sulfonate radical, for example, 5-sodium sulfoisophthalate, and copolymerized with a principal copolymerization component consisting of ethylene terephthalate.
The above-mentioned attempt is disadvantageous in that, where the metal sulfonate radical-containing isophthalic acid derivative is copolymerized in a necessary amount for enhancing the dyeability of the resultant copolyester fiber to a satisfactory level, with the principal copolymerization component, the resultant copolymerization product exhibits an excessively high melt viscosity due to the melt viscosity-increasing effect of the metal sulfonate radical-containing isophthalic acid derivative.
This makes it difficult to produce a copolyester resin having a satisfactorily high degree of polymerization. Also, the high melt viscosity copolyester resin causes a difficulty in melt-spinning when producing copolyester fibers.
Accordingly, it is necessary to reduce the melt viscosity of the copolyester resin containing the metal sulfonate radical-containing isophthalic acid derivative to an extent such that the copolymerization procedure for the copolyester resin can be easily carried out and the resultant copolyester resin can be readily melt-spun to form fibers. For this purpose, it is necessary to reduce the degree of polymerization of the copolyester resin, but the reduced degree of polymerization results in a decreased mechanical strength of the copolyester fibers. This decreased mechanical strength restricts the use of the resultant copolyester fibers capable of being dyed with cationic dyes. Japanese Examined Patent Publication (Kokoku) No. 47-22,334 and U.S. Pat. No. 3,732,183 disclosed a cationic dye-dyeable copolyester fiber consisting of a copolyester resin containing, as a copolymerization component for making the resultant copolyester fiber dyeable with cationic dyes, an isophthalic acid derivative having a phosphonium sulfonate radical.
The above-mentioned isophthalic acid derivative exhibits a low melt viscosity-increasing effect, and therefore, even if the resultant copolyester resin has a relatively high degree of polymerization, the melt viscosity of the copolymer resin is at a level such that the copolyester resin can be melt-spun in an ordinary melt-spinning method without difficulty. Accordingly, the copolyester resin can be used to provide cationic dye dyeable copolyester fibers having a high mechanical strength.
However, the above-mentioned copolyester resin is disadvantageous in that the thermal resistance of the phosphonium sulfonate radical-containing isophthalic acid derivative is lower than that of the metal sulfonate radical-containing isophthalic acid derivative. Accordingly, in the copolymerization step and melt-spinning step at a high temperature, the phosphonium sulfonate radical-containing isophthalic acid derivative component in the copolyester resin per se is decomposed, and sometimes promotes the thermal decomposition of the copolyester resin. The above-mentioned phenomena results in an undesirable discoloration (yellow brown) of the resultant copolyester resin or shaped article thereof and in a decrease in the degree of polymerization of the copolyester resin. Also, the discoloration of the copolyester fiber affects the hue of dyed copolyester fiber.
More specifically, the cationic dye dyeable copolyester fibers manufactured by the above-mentioned conventional methods have silk factors of 22 at the largest and scission numbers of 140.times.10.sup.23 at the smallest per 10.sup.6 g of fiber, and it has proved absolutely impossible to put these fibers to practical use.
Due to the above-mentioned disadvantages, the copolyester containing, as a copolymerization component, the phosphonium sulfonate radical-containing isophthalic acid derivative has not been industrially used.
The inventors have striven to obtain copolyester fibers which exhibit an improved stability. As a result, utilizing the methods described later in detail, the inventors have succeeded for the first time in obtaining copolyester fibers which exhibit silk factors of 25 or more and scission numbers of 140.times.10.sup.23 or less per 10.sup.6 g of fiber when wet heat-treated in a solution of 0.4 g/l of acetic acid in distilled water at a temperature of 130.degree. C. for 60 minutes, and thus have made it possible, for the first time, to put the cationic dye dyeable copolyester fibers to a practical use requiring strong mechanical properties, especially for uses that require strong mechanical properties and an excellent dyeing fastness when coated or laminated with finishing agents, for example, resin materials (the dye fixed to the fibers must not be transferred to the resin layer), i.e., when used in, sports wear, etc.